Hematoprophyrin derivative (Hpd) and tetraphenylporphine sulfonate (TPPS) have been shown to demonstrate the same kinetic and thermodynamic properties for both normal cells (L929, CHO) and malignant cells (HeLa, PC-1) under similar culture conditions. In culture, Hpd concentrates in cells, the equilibrium value depending upon extracellular protein concentration. The rate of incorporation of Hpd into a mouse tumor was equal to the rate of incorporation of albumin into the same tumor indicating that the Hpd-albumin complex may penetrate the "leaky" vasculature of the tumor. When injected into mice, Hpd accumulates to the highest levels in liver. The tumor/skin ratio is 2-3. Autoradiographic studies demonstrate very high levels of Hpd near the vascular stroma with lower levels distributed throughout the mass. Electron microscopic studies indicate that the Hpd initially accumulates in the cytoplasm of cells in a diffuse manner, followed by concentration in cytoplasmic vacuoles; possibly lysosomes.